A battery pack for electric power tool using a battery constituted of, for example, a lithium-ion rechargeable battery (hereinafter, also referred to merely as a “battery pack”) usually includes a control circuit which receives power supply from the battery to operate in order to implement various functions, such as controlling charge and discharge of the battery, monitoring the state of the battery, and so on.
Since there is a need to monitor the state of the battery even when the battery pack is not in use, the control circuit is generally configured to operate at all times by receiving power supply from the battery. Therefore, remaining amount of power of the battery gradually decreases even if electric power is not supplied to the body of the electric power tool. Battery capacity becomes empty in a relatively short period of time.
In contrast, a technique is known which completely cuts off power supply to the control circuit from the battery when the battery pack is not in use in order to make the battery last as long as possible (for example, See Patent Document 1 below).
Patent Document 1 describes that, when a load is disconnected from a storage battery, internal circuits in the storage battery are operated at low power consumption for a predetermined period of time and, after the predetermined time has elapsed, supply of energy fed to the internal circuits from cells in the storage battery is completely stopped so as to completely stop circuit operation.